To make coated cutting tools last longer, the performance of the coating is improved among other things. In connection with this, the coatings with an interstitial phase (interstitial alloy) hold out a special promise, for they possess hardness of a high order.
Known in the art is a wear-resistant coating for cutting tools and a method of applying same (cf. Patent of FRG No. 1,959,690; priority Nov. 28, 1969; IPC C23C 11/08, published Jun. 3, 1971) the interstitial phases whereof are carbon nitrides of the metals falling under the groups IVa-VIa of the Periodic Table of the Elements. The base metal of the tools given the above coating is a hard alloy, for example tungsten carbide (WC). The coating is deposited from a gaseous phase, and the base metal must be therefore heated to a temperature as high as 1000.degree. C. which may cause a hard alloy to lose some of its strength. However, the known coating cannot be applied by the known method to cutting tools the base metal whereof is a low-melting point one, e.g. steel. Said coating is based on carbon nitrides and contains one of the metals of the groups IVa-VIa of the Periodic Table of the Elements. To give the coating extra hardness and wear-resistance, alloying components may be introduced thereinto.
Also known is a wear-resistant coating for cutting tools having at least a single layer of an interstitial phase which is applied to the base and comprises alloying components as well as a non-metallic component the content whereof varies over the thickness of the coat (cf. Fizika i khimia obrabotki materialov, izdatelstvo Nauka, No. 2, 1979, pp. 169-170).
The interstitial phase formed, which is the coating, is based on molybdenum carbides and contains alloying components in the form of titanium, zirconium and tungsten. The main components of the interstitial phase, i.e. molybdenum and carbon react in the coating to form molybdenum carbide (Mo.sub.2 C) and molybdenum monocarbide (MoC). This means that a thermodynamically stable phase Mo.sub.2 C exists in a layer of coating together with an unstable one, e.g. MoC.
During machining, when the temperature of the cutting tool may rise to between 600.degree. and 1000.degree. C., the thermodynamically unstable compound of the heterogenous phase composition of the coating decomposes into the constituent elements (molybdenum and carbon), whereby Mo.sub.2 C is formed. This transformation taking place in the coating during machining sets up high stresses which render the coating brittle and susceptible to flaking and chipping. The service life of the tool is consequently reduced.
Further known is a method of applying a wear-resistant coating to a cutting tool which resorts to ionic bombardment in order to condense matter and consists in striking an electric arc in a vacuum to evaporate the material of a cathode, applying a bias voltage to the base metal of the cutting tool, heating up and cleaning the base metal by the bombarding ions of the vapourized cathode material, decreasing the bias voltage to a value whereat a layer of coating is formed, and lowering the temperature of base metal at the same time, feeding into the vacuum a gas which reacts with the evaporated cathode material to form a layer of coating of a given thickness which consists of an interstitial phase (cf. Fizika i khimia obrabotki materialov, izdatelstvo Nauke, No. 2, 1979, pp. 169-170).
The period elasped in depositing a layer of coating with a thickness of 5-8 .mu.m is between 45 and 60 min as a rule during the process of coating a tool. Since the reactivity of the main components of the interstitial phase decreases during this interval, thermodynamically unstable compounds are formed together with the stable ones. Moreover, since the temperature of depositing the coating is 420.degree. C. in this case, the diffusion of the gas into the base metal occurs at too high a rate. As a result, the content of the non-metallic component of the interstitial phase forming the coating is low, and thermodynamically unstable compounds detrimental to the strength of the coating are formed from the reaction with the evaporated cathode material.